Goldman Sachs cut its price target for Apple's stock to $165 a share from $187, making the firm's expectation for Apple the lowest of the major Wall Street banks, according to TipRanks.com.

The firm predicts 26% downside for the stock because of a "material negative impact" on earnings for the accounting method the iPhone maker will use for an Apple TV+ trial, Goldman analyst Rod Hall said in a note.

"Effectively, Apple's method of accounting moves revenue from hardware to Services even though customers do not perceive themselves to be paying for TV+," Hall said.

Goldman Sachs just significantly slashed its price target for Apple, predicting 26% downside for the shares because of a "material negative impact" on earnings for the accounting method the iPhone maker will use for an Apple TV+ trial.

"We believe that Apple plans to account for its 1-year trial for TV+ as a ~$60 discount to a combined hardware and services bundle," wrote Goldman analyst Rod Hall, in a note.

"Effectively, Apple's method of accounting moves revenue from hardware to Services even though customers do not perceive themselves to be paying for TV+. Though this might appear convenient for Apple's services revenue line it is equally inconvenient for both apparent hardware ASPs and margins in high sales quarters like the upcoming FQ1'20 to December," Hall added.

Apple shares fell as much as 2.6% in trading Friday after the note. Goldman cut its 12-month price target on the company to $165 from $187. The firm has a neutral rating on Apple's stock.

"We do not expect the introduction of Apple TV+, including the accounting treatment for the service, to have a material impact on our financial results," the company said in a statement to CNBC.

The stock rebounded slightly after Apple's statement but still finished trading down 1.9% at $218.75 a share.

Goldman is not accusing Apple of improper accounting but believes that hardware profit margins will suffer as a result of this TV+ free trial and investors will react negatively.

Hall explained that Apple has taken "a very similar approach" to its accounting methods before, for so-called embedded services such as Apple Maps and its Siri artificial assistant. The Goldman analyst expects the free trial TV+ revenue will add 25% to Apple's gross margin contribution, which, due to the lower product revenue, will result "in a negative calculated impact to EPS of 16%" for fiscal first quarter 2020, Hall said.

"We currently assume this is an introductory offer that runs for just one year. Should it run longer our out year forecasts would also likely need to be adjusted in a similar way," Hall added.

Here's an example of how Hall expects Apple's accounting will work in this instance. You buy a new iPhone 11 Pro for $1,000 (Hall rounded the $999 price to make the example easier). Apple accounts for the purchase as a bundle, as you're getting an iPhone 11 Pro and and a year of TV+, valued at $1,060. But the $60 discount for TV+ "is not solely apportioned to the TV+ revenue," Hall said, and Apple instead proportionally divides the discount to both.

Hall believes the combined discount is 5.7%, or $60 divided by $1,060. Applied to the bundle, the iPhone is discounted to $943.40 (or 94.3% of $1000) and TV+ annual price discounted to $56.60 (or 94.3% of $60). As a result, and assuming the iPhone is not bought with an installment plan, Hall found that, while the iPhone has a lower average price, it also comes at a lower profit to Apple because the cost of goods is not affected by the company's discount. However, at the same time, Hall expects the discounted TV+ revenue will be credited as "deferred revenue" and then "recognized on a monthly basis over the 12-month trial period," he said.